1. (Noun) - An YMY is a person who tries to behave much younger than they really are. They often shirk their duties as an adult or as a parent. They will try to do things that try to impress others, only succeeding in making themselves look foolish. An YMY will attempt to live vicariously through their children of their own or through other children.

2. (Verb) - Acting younger that you really are in a fruitless attempt to impress others.

She is suffering so badly from YMY, She wants her 7-year-old stepdaughter to start dieting with her.

If she weren’t such an YMY, She would focus more on her troubled son rather than getting plastic surgery.